The present disclosure relates to a communication apparatus and a communication method performing multicast transmission of the same data with respect to a plurality of receiving terminals, and in particular, to a communication apparatus and a communication method dynamically optimizing the multicast transmission rate, as well as a communication system.
As typical wireless LAN (Local Area Network) methods of communication, unicast communication transmitting data to a single terminal and multicast communication transmitting the same data to a plurality of terminals may be exemplified. In multicast communication, the same data may be transmitted at one time to a plurality of receiving terminals, whereby it is possible to reduce the use of transmission bandwidth. For example, in a case where a large amount of data such as a video is transmitted wirelessly, multicast communication is considered to be effective when the wireless communication transmission capacity is increased.
Here, in typical wireless communication, when the distance between transmission and reception is short (good communication quality), communication is performed at a high speed transmission rate, and when the distance between transmission and reception is long (poor communication quality), communication is performed after switching to a low speed transmission rate. For example, in a wireless LAN system including an access point (AP) and a client station (STA), a plurality of transmission rates are prescribed in order to communicate efficiently with a wide service area.
However, when performing multicast transmission of multicast data from the access point AP to a plurality of client stations configuring a multicast group, the slowest transmission rate is often selected. This is because multicast communication is different from unicast communication and there is no retransmission mechanism. Without information on whether a packet is transmitted correctly being returned from each client station, even if a packet from the access point AP is not transmitted correctly, it is difficult to perform retransmission.
FIG. 8 shows a situation of performing multicast transmission from one multicast transmission station (hereinafter, simply referred to as “transmission station”) to a plurality (four in the illustrated example) of multicast reception stations (simply referred to as “reception station”) 1 to 4.
For example, in wireless LAN systems in infrastructure mode, the access point AP becomes a multicast transmission station and each client station STA accommodated in a service area may become a multicast reception station. However, in a wireless system in which there is no control station such as an access point AP in ad-hoc mode or the like, arbitrary communication stations may become multicast transmission stations and multicast reception stations.
In the example shown in FIG. 8, the transmission station selects the lowest transmission rate R1 and the data signal (multicast data) is transmitted. Thus, the multicast data is delivered even at a reception station 3 for which the reception level is low at the end of the service area of the transmission station and for which the communication quality is poor. However, when multicast data of a large data size is generated at the transmission station, since the multicast data is buffered and the transmission rate is low, the time during which the wireless bandwidth is occupied becomes long. At this time, in a case where unicast communication such as of sound or images is performed between the transmission station and the other reception stations 1, 2, and 4 where the transmission quality is good, unicast data loss occurs, and a situation in which transmission efficiency is significantly reduced may occur.
On the other hand, when the multicast transmission rate is increased, since the time during which multicast transmission occupies the wireless bandwidth does not become long, the opportunity to transmit unicast data such as of sound or images is increased and loss is eliminated. FIG. 9 shows a situation of performing multicast transmission from one transmission station to a plurality of reception stations 1 to 4 at a transmission rate R2 (R2>R1) higher than the lowest transmission rate R1. In this case, the reception station 4 for which the reception level is low at the end of the service area of the transmission station and the communication quality is poor may not receive the multicast data at the transmission rate R2. In addition, since the reception station 4 does not return a reception acknowledgement ACK corresponding to the multicast data, the transmission station may not detect non-delivery of the multicast data due to the degradation of the communication quality.
In short, when the multicast transmission rate is fixed at the slower rate side, the time during which the wireless bandwidth is occupied becomes long, whereby there is a problem in that the overall transmission efficiency of the system is deteriorated. Conversely, when the multicast transmission rate is fixed at the higher rate side, there are problems in that the communication quality is degraded and the non-delivery of the multicast data at the reception station may not be detected.
Incidentally, a transmission station apparatus which dynamically sets the multicast transmission rate and a wireless communication system have been proposed. The transmission station apparatus periodically transmits a pseudo training signal at a predetermined multicast transmission rate using unicast or multicast to each of a plurality of transmission station apparatuses and receives ACK responses from the plurality of transmission station apparatuses with respect to the pseudo training signal. Then, the transmission station apparatus varies the multicast transmission rate according to the presence or absence of the ACK responses from the plurality of reception station apparatuses and repeats the unicast transmission of the pseudo training signal. When there are ACK responses from all of the plurality of reception station apparatuses, the multicast transmission rate is selected as a predetermined multicast transmission rate (for example, refer to Japanese Unexamined Patent Application Publication No. 2010-161554).
In the above wireless communication system, when a pseudo training signal is transmitted by unicast transmission, even in a state where the communication quality is degraded, there are times when data is delivered to the reception station apparatus by the retransmission control function. As a result, it is determined that the communication quality is better than it actually is, whereby there is a tendency to select a high multicast transmission rate. Meanwhile, the retransmission control in the multicast transmission typically does not function. Since the pseudo training signal is delivered to the reception station apparatus according to this difference, the unicast transmission rate at that time is selected as a predetermined multicast transmission rate. In such a case, even when transmission is performed using multicast, a situation in which the multicast data is not delivered may occur.
On the other hand, in a case where the pseudo-training signal is transmitted by multicast transmission, the transmission is only performed periodically at a predetermined cycle. Therefore, in a wireless transmission path in which link loss is generated at a certain frequency, a situation may occur in which a pseudo training signal may not be received due to the momentarily poor communication quality. As a result, even though the bandwidth is sufficient, a low multicast transmission rate may be set, and there is a problem in that the overall transmission efficiency of the system is deteriorated.
In addition, in the wireless communication system described above, even when there is a priority among the plurality of reception stations, if degradation of the communication quality occurs in part of the reception station apparatus where the priority is low, the multicast transmission rate is deteriorated even with respect to many other reception station apparatuses in a favorable communication environment.
In addition, in the above wireless communication system, the channel quality is estimated using the pseudo-training signal. For this reason, other than at the moment of training, the transmission station apparatus may not dynamically optimize the multicast transmission rate in relation to changes of communication quality or the like caused by ongoing variations of the propagation environment on the reception station apparatus side. In other words, even if there are variations in the communication environment during multicast transmission at each reception station configuring the multicast group, the transmission station apparatus may not correspond thereto.